2018 U of MN Cooperative Black Cutworm Trapping Network Report #4

Week of May 5-11, 2018

This newsletter is available in a print-friendly pdf format: 2018 Black Cutworm Network Report #4

Southern MN black cutworm moth captures. May 5-11Figure 1. Maximum 2-night black cutworm moth captures by county. May 5-11, 2018.

Once again, the past week brought more rain and black cutworm (BCW) moths to many trap locations. Typically, at this time of year, a larger percentage of fields in southern Minnesota have been worked, and thus are less attractive to moths. Both corn and soybean planting are typically further along than they are this year.

The reported maximum 2-night moth capture for the most active trap location for each county during the week are shown in Figure 1.

Table 1 shows counties that reported significant (numbers indicating potential risk for economic damage to row crops) trap captures and dates. It also presents the degree-day calculations that predict egg hatch and beginning of leaf feeding, the start of the 4th instar (stage) larvae, which are large enough to cut small corn plants, and when cutting stops and pupation begins.

Traps in the southern tier of counties in SW and SC MN continue to be very active. This is also some of the area that has been suffering from excessive moisture. The traps in Dodge and Mower County have not been reporting but I suspect they would have similar captures to neighboring counties if weather was similar. There may have been undetected moth activity in Pipestone and Lincoln Counties but traps were lost during a storm. In any event, moth captures have been widespread in southern MN. Why don’t we have more traps in northern MN? Most often, spring weather systems that would bring BCW moths from the south are cut off in the central part of the state, often bending east of even our southern counties. This year is a bit of an exception with moths captured further north. Even here, the few moths that were captured in Polk County are likely short-distance rather than long-distance migrants. In Minnesota springs, the long-distance migrations typically contain many more individuals. Other species of cutworms overwinter and should not be overlooked when scouting, even if an area has not received a large influx of BCW moths.

BCW eggs from the earliest significant flights are now hatching, and the small larvae initially feed on leaves of weeds and emerged crops. This leaf feeding will be the first evidence that cutworms are present. As they grow, the larvae will start to cut small weeds and eventually corn and soybean seedlings. The larvae will be large enough to cut corn by the last few days of May and early June. Of course, the development of larvae from later flights will be a bit later.

So…. how do you make your scouting efforts more productive? The following criteria produce increased risk for black cutworm infestation and potential stand loss. Target these highest risk fields first.

  1. Fields without spring tillage before a significant flight arrives. Soybean residue is often preferred.
  2. Winter annual or early-emerging annual weeds and live cover crops may be attractive as well. Ridge and strip till do not disturb residues, and eggs between rows and warmer, bare soil can allow more early-season spring annual growth than no-till.
  3. Corn without a Herculex I or Viptera aboveground Bt trait. These hybrids can still be damaged when large numbers of large, late instar cutworms move from feeding on weeds to small corn.
  4. At-plant rootworm insecticide not used. 

Cutworms are relatively easy to scout for and control with targeted post-emerge insecticides. Initially, scouting will focus on finding leaf feeding. As cutting begins, species ID and treatment thresholds become important for making economically sound decisions.

Things continue to favor of black cutworm and not corn this spring! Small corn is more susceptible to cutting and the later corn planting this year will lead to more corn acres in a susceptible stage. Over the next few weeks, we will try to fine-tune temperature-based predictions for when cutworm cutting should begin and end.

In the meantime, you can find much more black cutworm information at: https://www.extension.umn.edu/agriculture/corn/pest-management/black-cutworm/. This fact sheet provides information on what makes a field attractive to black cutworms, cutworm growth and development and tips on cutworm scouting and management. Pay close attention to the sections on scouting, cutworm development and economic thresholds as your corn scouting begins.

Table 1. Significant moth flight capture locations, dates and larval development predicitons.
County 2-night capture Biofix date Projected leaf feeding1 Projected start of cutting2 Projected end of cutting3
Blue Earth 8 3-May 13-May 1-Jun 21-Jun
Dakota 8 8-May 18-May 6-Jun 24-Jun
Faribault 10 8-May 17-May 4-Jun 22-Jun
Jackson 11 8-May 18-May 5-Jun 22-Jun
Martin 20 2-May 11-May 1-Jun 20-Jun
Martin 12 9-May 18-May 4-Jun 23-Jun
McLeod 10 4-May 14-May 1-Jun 21-Jun
Murray 9 2-May 13-May 1-Jun 21-Jun
Nobles 27 1-May 12-May 1-Jun 21-Jun
Olmsted 8 2-May 12-May 2-Jun 22-Jun
Pipestone 9 4-May 15-May 2-Jun 23-Jun
Redwood 8 3-May 12-May 30-May 19-Jun
Rock 11 3-May 13-May 31-May 20-Jun
Rock 8 9-May 20-May 6-Jun 24-Jun
Rock 14 10-May 21-May 7-Jun 25-Jun
Rock 21 11-May 21-May 9-Jun 25-Jun
Steele 11 9-May 22-May 7-Jun 25-Jun
Swift 9 7-May 17-May 4-Jun 23-Jun

1Based on 90 degree-days (base 50oF) after significant flight (leaf feeding begins).

2Based on 312 degree-days (base50oF) from significant flight. 4th-6th instar larvae are large enough to cut corn. Small plants, e.g. sugarbeets, can be cut earlier.

3Based on >641 degree-days (base 50oF) after significant flight pupation.

Previous black cutworm reports can be found at https://swroc.cfans.umn.edu/agricultural-programs/pest-management/black-cutworm-reporting-network .

Thanks to all of the cooperators who are maintaining traps and Minnesota Corn Research and Promotion Council for support. Because the delay in spring tillage has prolonged the length of time that fields are attractive to egg-laying moths, the cooperative trapping network will run at least one more week, more if cooperators are willing.

If you find black cutworm damage in your corn, soybean, sugarbeet and other crop scouting this spring, we would like to hear about it. When they are compared to trap results, field reports of cutworm size and population density, it can help improve our predictions.

Until next week,  

Bruce Potter and Travis Vollmer